1. Field of the Invention
The present invention relates generally to electronic equipment with controlling and controlled units, and more particularly to an apparatus for controlling power and signal of electronic equipment with a controlling unit and a subordinate unit.
2. Description of the Related Art
In general, electronic equipment has many functional sections to which subordinate modules like a controlling unit, an inputting unit, or a processing unit are connected. One example of a piece of electronic equipment having a controlling unit and a subordinate module is cellular phone. Usually, one can save power consumption or increase battery usage time by turning off power supplied to the subordinate module that is not used at a certain time.
FIG. 1 is a block diagram showing an apparatus for controlling power and signal of a piece of electronic equipment having a controlling unit and a subordinate module in accordance with a related art. Referring to FIG. 1, connected between controlling unit 2 and subordinate module 4 is a signal control line 6 for authorizing operation control signals to subordinate module 4, and a power control line 8 for authorizing power control signals. A resistor R1 is serially connected onto signal control line 6, and connected to one end of capacitors C1, C2, and C3, having their other ends being connected to ground. Particularly, capacitor C1 is for removing noise on the signal control line 6, capacitor C2 is a virtual capacitor for indicating capacitance elements existing on signal control line 6, and capacitor C3 is also a virtual capacitor for indicating capacitance elements of subordinate module 4.
FIG. 2 shows waveforms of each unit in FIG. 1. More specifically, reference (a) indicates a waveform of the power control signals at subordinate module 4 input end, (b) indicates a waveform of the operation control signals at controlling unit 2 output end, and (c) indicates a waveform of the operation control signals at subordinate module 4 input end.
As shown in the drawing, power of subordinate module 4 is turned off for the time interval A (t0˜t1, t4˜) where the power control signal sent to subordinate module 4 is at a ‘low’ state, while power of subordinate module 4 is turned on for time intervals B (t1˜t2, t3˜t4) and C (t2˜t3) where the power control signal sent to subordinate module 4 is at an ‘active high’ state. However, during A interval, namely when the power control signal is at a ‘low’ state and the operation control signal is at a ‘high’ state, the signal level of operation control signal at ‘high’ state, e.g., 3V, often causes leakage current toward subordinate module 4 even though resistor R1 is connected onto signal control line 6 to prevent leakage current in some degree.
Leakage current through subordinate module 4 via the signal control line during the time interval A is determined by the voltage difference between the two ends of resistor R1 and the resistance value for the time interval A shown in FIG. 2(c). In short, leakage current is calculated by using the formula: (3V−1V)/R1. Moreover, the relation between the resistance value of resistor R1 and the impedance value of the signal control line 6 of subordinate module 4 during the time interval A determines the operation control signal level of subordinate module 4 at the input end. For instance, if resistance value of resistor R1 is increased, the voltage across resistor R1 is increased, relatively lowering the operation control signal level of subordinate module 4 at the input end. However, the degree of any decrease in the operation control signal level is relatively small, compared with the degree of the increase in resistor R1, such that it can be disregarded. Nevertheless, there is a limit to reducing leakage current by setting resistor R1 infinitely large. The reason is that when resistor R1 value is set infinitely large, the operation control signal level to be input to subordinate module 4 cannot be disregarded any more.
In addition, time constant is affected by resistor R1 value and a CT value, the composite capacitance of three capacitances existing on signal control line, i.e., CT=C1+C2+C3. If resistor R1 value is set high, the time constant is increased. As a result, waveforms of the operation control signals of subordinate module 4 at the input end are distorted as depicted in FIG. 2(c). Further, as the same drawing manifests, the distorted waveform during the time interval C causes error with operation of subordinate module 4. This means that resistor R1 value and CT value should be lowered to reduce the degree of distortion in waveforms of the operation control signal of subordinate module 4 at the input end. However, CT value is not a definite value because of C2 and C3 elements, and there is a limit to lower that value. This leaves only one option to lower resistor R1 value to reduce distortion in waveforms of the operation control signals of subordinate module 4 at the input end. Unfortunately though, if resistor R1 value is lowered, one cannot effectively prevent leakage current toward subordinate module 4 through signal control line 6 during the time interval A.